Effect of Initial Stress Difference on Hard Rock Fracture Mechanism Under True Triaxial Excavation Unloading Stress Paths

The deep excavation unloading stress path and initial high stress difference (sigma 20-sigma 30\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sigma_{2}<^>{0} - \sigma_{3}<^>{0}$$\end{document}) are distinctive features that differentiate deep engineering from shallow engineering. To investigate the mechanism of deep hard rock failure induced by initial stress differences under excavation unloading stress paths, a series of typical true triaxial unloading tests are conducted using a true triaxial experimental system. The results show that as the initial stress difference increases, there is a gradual enhancement in both the strength, brittleness and ultimate energy storage capacity of the rock. Additionally, the direction of rock fracture propagation gradually aligns with sigma 2, leading to an increase in the macroscopic failure angle of the rock. Moreover, the roughness of the rock fracture surface decreases, while transgranular and tensile fractures become more prevalent, often exhibiting sudden, high-energy fracture events. Building upon the non-uniform stress distribution characteristics induced by the initial stress difference, a principle governing the differential development of rock fractures is proposed. This principle underscores the pivotal role of the initial stress difference in triggering spalling of the surrounding rock parallel to the sidewall. Furthermore, a functional relationship between the initial stress difference and the propensity for rockburst events is established. These findings hold significant implications for understanding the failure mechanisms of surrounding rock in deep hard rock engineering excavations and for evaluating the risks associated with spalling and rockburst hazards. The failure mechanism of hard rock caused by the initial stress difference under the true triaxial excavation unloading stress path is revealed.The principle of differential development of rock fracture induced by initial stress difference is proposed, which reveals that initial stress difference is the decisive factor for the spalling of surrounding rock parallel to the side wall.The increase of initial stress difference can improve the rockburst tendency of rock, and the relationship between initial stress difference and rockburst tendency index is the exponential function.